QUIZ 4~CH. 25
EXERCISES/PROBLEMS
6, 7, 26, 28, 67, 30, 34, 35, 36, 37, 32, 71,  33, 51, 53, 55
 6. (a) Number of atoms =(mass/molar mass)*NA, where NA = Avogadro's number and mass = density*volume.   Density = 8900 kg/m3  and volume = 1.00 m3. Molar mass = 0.064 kg/mole. 
(b) You already  know  the #  atoms/m3 from part (a). 
( # free electrons)/atom = (# free electrons/m3)/(# atoms/m3)      
7. (a) Integrate the current formula; you will obtain an expression containing t3. Evaluate at the two limits of time and subtract to get  definite integral. 
(b) Q = I*time, where I is the contain current you seek. Set this value of Q to the part (a) result ~~~ Find I.   
 26.  I/A = J = 4.40x10 7 A/m2 =   E/resistivity   = V/(L*resistivity) using E = V/L, where V is the magnitude of the voltage difference between the two points in space.    Find E.
 28. See pages  852 and 854 on temperature dependences; we assume area A and length L do not change appreciably during the heating process . See problem 67, discussed next, for a counter example. 

 67. I wrote the hints to this problem on the white board at least twice in the last two weeks. 
(a) Use equation 25. 10; get area A from the formula for a circle with given diameter.
(b) See equation 25.6.
(c) This question is "off topic" and will be discussed in detail in class. 
(d) change in R =
(dR/dL)*(change in L)+  
(dR/dA)*(change in A)+
(dR/d rho)*(change in rho ),

where rho = resistivity,

We are given that the change in A = 0 so the second term in the above sum is ZERO. MORE DISCUSSIONS LATER. 
 MORE DISCUSSIONS LATER.